how did the two friends discover the truth about the trout in three men in a boat part 2

There is a large trout hanging on the wall there, and three different patrons (plus the bartender) each claim they were the one to catch it, each with a different story and description of its weight. At the end of the night, George trips and grabs the trout to steady himself. The trout falls to the ground and shatters, and the men realize that it is made of plaster of Paris.